The Origin of Life

The Origin of Life is Hypothesis Special Creation Was life created by a supernatural or divine force? not testable Extra-terrestrial Origin Was the original source of organic (carbon) materials comets & meteorites striking early Earth? testable Spontaneous Abiotic Origin Did life evolve spontaneously from inorganic molecules?

Early conditions on Earth Reducing atmosphere water vapor (H2O), CO2, N2, NOx, H2, NH3, CH4, H2S lots of available H & its electron no free oxygen Energy source lightning, UV radiation, volcanic It is unclear whether young Earth’s atmosphere contained enough methane and ammonia to be reducing. Growing evidence suggests that the early atmosphere was made up primarily of nitrogen and carbon dioxide and was neither reducing nor oxidizing (electron–removing). Miller–Urey–type experiments using such atmospheres have not produced organic molecules. Still, it is likely that small “pockets” of the early atmosphere—perhaps near volcanic openings—were reducing. Instead of forming in the atmosphere, the first organic compounds on Earth may have been synthesized near submerged volcanoes and deep–sea vents—weak points in Earth’s crust where hot water and minerals gush into the ocean.

Conditions on Early Earth Reducing atmosphere water vapor (H2O), CO2, N2, NOx, H2, NH3, CH4, H2S lots of available H & its electron no free oxygen Energy source lightning, UV radiation, volcanic low O2 = organic molecules do not breakdown as quickly It is unclear whether young Earth’s atmosphere contained enough methane and ammonia to be reducing. Growing evidence suggests that the early atmosphere was made up primarily of nitrogen and carbon dioxide and was neither reducing nor oxidizing (electron–removing). Miller–Urey–type experiments using such atmospheres have not produced organic molecules. Still, it is likely that small “pockets” of the early atmosphere—perhaps near volcanic openings—were reducing. Instead of forming in the atmosphere, the first organic compounds on Earth may have been synthesized near submerged volcanoes and deep–sea vents—weak points in Earth’s crust where hot water and minerals gush into the ocean.

Earth = 4.6 billion years old First life forms appeared ~3.8 billion years ago How did life arise? Small organic molecules were synthesized Small molecules  macromolecules (proteins, nucleic acids) Packaged into protocells (membrane-containing droplets) Self-replicating molecules allow for inheritance “RNA World”: 1st genetic material most likely RNA First catalysts = ribozymes (RNA)

Synthesis of Organic Compounds on Early Earth Oparin & Haldane: Early atmosphere = H2O vapor, N2, CO2, H2, H2S methane, ammonia Energy = lightning & UV radiation Conditions favored synthesis of organic compounds - a “primitive soup”

Tested Oparin-Haldane hypothesis Simulated conditions in lab Miller & Urey: Tested Oparin-Haldane hypothesis Simulated conditions in lab Produced amino acids produced -amino acids -hydrocarbons -nitrogen bases -other organics

Protocells & Self-Replicating RNA

Fossil Record: used to reconstruct history Sedimentary rock (layers called strata) Mineralized (hard body structures) Organic – rare in fossils but found in amber, frozen, tar pits Incomplete record – many organisms not preserved, fossils destroyed, or not yet found

Both used to date fossils and determine age Relative Dating Radiometric Dating Both used to date fossils and determine age Uses order of rock strata to determine relative age of fossils Measure decay of radioactive isotopes present in layers where fossils are found Half-life: # of years for 50% of original sample to decay

Clock Analogy of Earth’s History

Key Events in Life’s History Origin of Cells (Protobionts) lipid bubbles  separate inside from outside  metabolism & reproduction Origin of Genetics RNA is likely first genetic material multiple functions: encodes information (self-replicating), enzyme, regulatory molecule, transport molecule (tRNA, mRNA) makes inheritance possible makes natural selection & evolution possible Origin of Eukaryotes endosymbiosis The earliest living organisms were prokaryotes. About 2.7 billion years ago, oxygen began to accumulate in Earth’s atmosphere as a result of photosynthesis. Eukaryotes appeared about 2.1 billion years ago. Multicellular eukaryotes evolved about 1.2 billion years ago. The colonization of land occurred about 500 million years ago, when plants, fungi, and animals began to appear on Earth. O2 accumulates in atmosphere (2.7 bya) Humans (200,000)

Endosymbiont Theory Mitochondria & plastids (chloroplasts) formed from small prokaryotes living in larger cells Evidence: Replication by binary fission Single, circular DNA (no histones) Ribosomes to make proteins Enzymes similar to living prokaryotes Two membranes Evidence structural mitochondria & chloroplasts resemble bacterial structure genetic mitochondria & chloroplasts have their own circular DNA, like bacteria functional mitochondria & chloroplasts move freely within the cell mitochondria & chloroplasts reproduce independently from the cell

Pangaea = Supercontinent Formed 250 mya Continental drift explains many biogeographic puzzles Continental drift is movement of Earth’s continents on great plates that float on the hot, underlying mantle. The San Andreas Fault marks where two plates are sliding past each other. Where plates have collided, mountains are uplifted. Continental drift can help explain the disjunct geographic distribution of certain species, such as a fossil freshwater reptile found in both Brazil and Ghana in west Africa, today widely separated by ocean.

Mass extinctions  Diversity of life Mass extinctions, loss of large numbers of species in a short period, have resulted from global environmental changes that have caused the rate of extinction to increase dramatically. Adaptive radiations are periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological niches. Example: Galapagos finch species. Major periods in Earth’s history end with mass extinctions and new ones begin with adaptive radiations

Major events during each Era Precambrian: microscopic fossils (stromatolites) Photosynthesis, atmospheric O2 Eukaryotes (endosymbiont theory) Paleozoic: Cambrian Explosion Plants invade land, many animals appear Permian Extinction (-96% species) Mesozoic: “Age of Reptiles”, dinosaur, plants Formation of Pangaea supercontinent Cretaceous Extinction – asteroid off Mexico’s coast Cenozoic: Primates Note: All end with major extinction & start with adaptive radiation

Need Help? Here are Some Extra Resources: Bozeman Biology: Origin of Life Video PBS NOVA: Origin of Life